Over 200,000 people die annually in the US from the consequences of sepsis. While molecularly guided therapy has been introduced for many diseases, such interventions remain elusive in the treatment of sepsis. Many patients that survive the acute phase of sepsis develop secondary infections and succumb to these later complications. The majority of septic patients are mechanically ventilated, and sepsis is an independent risk factor for ventilator associate pneumonia (VAP). VAP is the most common nosocomial infection in the ICU, is responsible for the majority of ICU antibiotic use and carries a mortality of close to 40%. While factors that interrupt normal upper airway defense mechanisms are important contributors, host immune factors likely play an equally critical role. The identification of these factors is a major goal this proposal. Importantly, these pathways may offer novel opportunities for therapeutic manipulation. Recent work has documented a state of relative immunosuppression occurring in both animal models and humans with sepsis. Apoptotic cell death of lymphocytes as well as a defect in the function of the remaining cells has been demonstrated, of which the underlying mechanism has not been determined. Prevention or reversal of this immunosuppression would be predicted to prevent many of the later sequelae of sepsis and thereby improve patient survival. During a normal immune response lymphocyte function is restricted by specific receptor mediated, inhibitory signaling pathways and by the action of distinct cell types including regulatory T (Treg) cells and myeloid derived suppressor cells (MDSC). We hypothesize that in sepsis there is an inappropriate, widespread induction of suppressive pathways resulting in the active inhibition of T cell function. We propose that increased expression of inhibitory receptors on peripheral blood lymphocytes and/or an increase in suppressor cell populations (Tregs and MDSCs) will identify and differentiate those patients at increased risk of secondary infection from those not at risk, and thereby serve as a "biomarker" allowing for targeted therapies. We also propose that the susceptibility to VAP is driven by the parenchymal cells of the lung. By expressing specific inhibitory ligands and metabolic pathways such as indoleamine 2,3 dioxygenase (IDO), the lung parenchyma creates an immunosuppressive environment that actively inhibits lymphocyte function. We propose to test this hypothesis by characterizing the blood and lung cells from with patients sepsis that develop VAP admitted to the ICU's at Barnes-Jewish Hospital. While exploratory, these studies have the potential to be paradigm shifting and lay the groundwork for novel therapeutic interventions in sepsis. Currently biologics that manipulate CTLA4, PD-1 and IDO signaling are in clinical trials as immune modulating agents in cancer patients. The results of this study will allow for the clinical identification of immunosuppressed patients at risk for infection, as well as provide preliminary data in support of a larger, definitive trial with the eventual goal of a randomized intervention trial. PUBLIC HEALTH RELEVANCE: This proposal aims to understand the reason for the suppressed immune response in critically ill patients. The relative immunosuppressed state in these patients often results in infections that are the cause of death. The studies we propose in this application will allow us to identify patients at risk for infection, and design treatments to enhance their immune function.